RAR Expander is a free and open source Mac OS X program that extracts the files contained in RAR archives. RAR Expander supports both single and multi-part archives, and has support for password-protected archives as well.
RAR Expander uses the official unRAR library internally so it is fully compatible with archives produced by WinRAR.
RAR Expander also features AppleScript support, and includes a few useful example scripts for expanding multiple archives at once.